1. Field of the Invention
The invention relates to a driving method and apparatus for plasma display panels (PDP) with variable vertical frequency, more particularly to a driving method and apparatus for increasing sustain period in PDPs to gain high brightness.
2. Description of the Related Art
According to current driving method for PDPs, a frame includes several subfields to complete an image display. To represent 256 gray scales, for example, 8 subfields can be used to complete an image display in a predetermined order, as shown in FIG. 1. A complete frame contains 8 subfields sequentially from SF1 to SF8, each consisting of reset period (R1˜R8), address period (A1˜A8) and sustain period (S1˜S8). In the reset period, residual charges of previous frame and a certain number of wall charges in every display cell are reset. In the address period, wall charges are accumulated in display cells to be activated (in “on” state) through address discharge. In the sustain period, display is continued by sustaining discharges in the cells to be activated. In the reset periods R1˜R8 and the sustain periods S1˜S8, display cells of a PDP are concurrently processed, whereas in the address periods A1˜A8, display cells on a scan line are sequentially processed, line by line. Display brightness is positive in proportion to the lengths of the sustain periods S1˜S8. In this case, the length of the sustain periods S1˜S8 in the subfields SF1–SF8 are proportionally weighted to values 1:2:4:8:16:32:64:128 that indicate required numbers for sustaining discharge cycle to represent 256 gray scales.
When duration of the sustain period is increased, PDPs show a higher brightness. Accordingly, if the sum of the sustain periods in a frame of a PDP is represented by ΣTsustain , a reset action in one subfield of the frame is set to Tr, a write action in one scan line of the subfield is set to Ts, the number of scan lines of the frame is set to Ns, and each frame of the PDP has the number Nsf of subfields, the display time for one frame of the PDP T (=1/vertical frequency of the PDP) can be represented by:T=(Tr+Ts*Ns)*Nsf+ΣTsustain.
To obtain high brightness for current PDPs by means of the ΣTsustain increase, two methods are adopted:
1. The number of subfields for one frame is changed, i.e., changing Nsf such that high brightness is obtained by adjusting subfield number and total sustain pulses in the PDP through image loading change. However, this way causes poor image quality due to fewer subfields in high brightness.
2. The number of scan lines is changed, i.e., changing Ns to increase total sustain pulses in the PDP such that high brightness is obtained due to increased total sustain pulses in the PDP. However, this way causes poor image quality for low gray-scaled frame display.